1. Field of the Invention
This invention relates to an extruder screw and, more specifically, to an extruder screw for use with apparatus for preparing and processing a cooked, farinaceous mash. In particular, the present invention relates to an extruder screw for use with apparatus for continuously pressure cooking, extruding and forming pellets of a food product and, in which, the apparatus includes a pressure feeder assembly, a seal plate assembly, a pressure cooker assembly and an extruder assembly, which is then normally followed by a cutoff assembly for pelletizing the extruded product.
2. Description of the Prior Art
The present invention is an improvement in and for applicants' assignee's cooker/extruder apparatus which is fully disclosed in U.S. Pat. No. 3,246,594 to C. D. Fisher, the disclosure of which is intended to be incorporated herein by reference.
Briefly, disclosed in the aforesaid U.S. Pat. No. 3,246,594 is an apparatus for continuously pressure cooking various cereal materials, under superatmospheric steam conditions, to produce a cooked, extrudable mash and, then, extruding the cooked mash through a plurality of die orifices. The fabricated extruder screw, shown in the patent, is essentially a double flight screw, in which, within the screw barrel, the flight sections are interrupted or cut at several locations to provide rectangular slots in or through the flights to promote intermingling of the mash carried in the respective flight channels. Also, shown in part in the patent, are an additional pair of flight segments at the terminal end of the screw for the purpose of enhancing more uniform distribution of the mash at the extruder die.
In general, a double flight screw of screw section, as shown in the patent, is selected because it discharges product in a more uniform stream than a corresponding single flight screw or screw section. However, difficulties were encountered with the double flight screw design of the patent in getting a wide variety of mash materials of differing viscosity to move at a desired uniform rate into and through the extruder barrel. Accordingly, the fabricated extruder screw design was modified to include a single flight section across the extruder inlet, followed, in series, by a double flight section extending through the major portion of the barrel, a flightless section provided with lugs for mixing the mash and a second, relatively short, double flight section at the terminal end of the screw.
The above fabricated construction has worked satisfactorily over a number of years. However, recently attention was given to substituting machined screws for the prior art fabricated screws, primarily for reasons of improved screw strength and screw flight uniformity, as well as potential cost savings. In so doing, a number of screw designs were considered, it being understood that screw design is as much, or more so, a matter of art and experience, rather than a strict application of precise scientific principles. From this activity evolved the present screw design which, surprisingly, when placed in an actual production situation and with no change in any other operating parameters, significantly increased the throughput of the extruder, while fully maintaining a high standard of product quality.